1. Field of the Invention
The present invention relates to devices for measuring pressure and more particularly, to devices which utilize optical means for the measurement of pressure.
2. Description of the Prior Art
Rocket engines for space missions beyond the present decade will be required to operate for longer periods of time, withstand many more firing cycles and offer more reliability than present-day engines. Such improved performance, however, will prove obtainable only if direct, on-board instrumentation can be provided to monitor pressures at certain critical locations within the engine. With such provisions, engine operation can be more effectively controlled for purposes of efficiency, endurance and safety.
For a pressure probe to be suitable for in-flight application, it must not only provide highly responsive and accurate pressure readings, but also it must be small and lightweight, yet capable of withstanding the rigorous environment of the rocket engine. Heretofore, pressure sensors have proven limited for in-flight application, for even the most advanced prior art pressure sensors such as those used in the Space Shuttle Main Engine ("SSME") provide only limited performance and add unacceptable amounts of weight and complication to the engine system.
The pressure sensors on the SSME generally comprise a remote pressure transducer, a pressure tap at the location of interest and a pressure tube for connecting the two. A typical construction for the transducer employs a pressure deflectable diaphragm and a strain gauge affixed thereto for generating signals which are then applied to a balanced bridge-type of a circuit. These sensors offer only limited responsiveness because the pressure tube dampens out the pressure pulses picked up by the pressure tap and the responsiveness is so affected that these probes are suitable for reading only steady state conditions. This limitation could of course be improved if the transducer were mounted directly at the location of interest; however, these locations are usually high-temperature regions whereat the transducers cannot operate properly without complicated cooling arrangements. Even when these transducers incorporate temperature resistent materials, their performance becomes wholly unacceptable at temperatures above 1300.degree. F. Thus the strategy has remained to link the transducers to the points of measurement by means of pressure tubes to thereby thermally isolate the sensor and consequently these pressure sensors have remained useless for measuring transients, instabilities and other conditions necessary for effective engine monitorization. Moreover, their installation requires the addition of 20 pressure tubes and forty extra flanges to the SSME engine system, adding greatly to its weight and complexity.
There is another class of pressure sensors which utilize a reflective, pressure deflectable diaphragm and a bundle of optical fibers whose end is positioned over the center of the diaphragm. Some of the optical fibers of the bundle serve as a source of an incident beam which strikes the reflective diaphragm to form a reflected beam. The reflected beam is then received by the remaining portion of the optical fibers and the characteristics of the two beams are compared. Because the deflection of the reflective surface changes the length of the optical path of the two beams, the comparison of such parameters as the beams' relative intensities or width can be related to the deflection of the surface, which changes can then be correlated to the level of pressures causing the subject deflections. However, these devices suffer from the crippling disadvantage that the bundle of optical fibers needs to be placed extremely close to the reflective diaphragm (on the order of one millimeter). This requirement subjects the optical fibers to any heat transferred to the reflective diaphragm, which exposure can affect the performance of this type of sensor and will invariably cause it to fall at elevated temperatures.